A Passive Optical Network (PON) is a type of an optical access network, which allows a plurality of users to be connected to a node of a core network that is for instance a metropolitan area network.
A PON typically comprises at the interconnection of the passive optical network and the core network an Optical Line Termination (OLT). The OLT is considered as a part of the PON. The OLT is connected to an Optical Distribution Network (ODN), which is also a part of the PON. The ODN comprises a plurality of optical links and passive optical components arranged so as to form a point to multipoint structure whose root is connected to the OLT.
The OLT is typically located at the so called Central Office (CO) of the service provider. Each optical link of the OLT may be terminated at its far end by a respective Optical Network Unit (ONU).
In a WDM PON, each ONU may communicate with the OLT, by using a respective set of wavelengths comprising an upstream wavelength, which the ONU uses for data transmission from the ONU to the OLT, and a downstream wavelength, which the OLT uses for transmission from the OLT to the ONU.
In a WDM PON, the ODN typically comprises a so called remote node, a feeder optical fiber connecting the remote node to the OLT and a number of distribution optical fibers radiating from the remote node. Each distribution fiber may be terminated at its far end by an ONU or by a power splitter connecting the distribution fiber to the multiple ONUs via respective optical drop fibers.
In the downstream direction, the OLT generates downstream optical signals at the downstream wavelengths associated to the ONUs using respective transmitters. The OLT multiplexes the different downstream signals according to the known WDM technique and transmits them to the remote node along the feeder fiber. At the remote node, the downstream signals may be de-multiplexed by an optical de-multiplexing device onto the distribution fibers and thus transmitted to the respective ONUs along the respective distribution fibers. Alternatively, the downstream signals may be split in power at the remote node onto the distribution fibers using an optical splitting device and thus transmitted towards the ONUs via the distribution fibers.
In the upstream direction, each ONU generates a respective upstream signal at the respective upstream wavelength associated thereto and transmits this signal along the respective distribution fiber to the remote node. The remote node multiplexes the upstream signals received from the various ONUs via the different distribution fibers according to the known WDM technique and forwards them to the OLT through the feeder fiber. At the OLT, the upstream signals are de-multiplexed and provided to respective receivers.
The remote node may be a passive node, which means, that the remote comprises only passive optical components for de-multiplexing or splitting the downstream signals and multiplexing the upstream signals.
Due to the fact that the different optical signals transmitted within a PON are subject to the effect of attenuation caused by the optical fibers of the PON as well as possibly the effect of power reduction due to power splitting, there is a need to counteract these effects. A solution known from the prior is to foresee within the PON an amplification device that is preferably called reach extender. Such an amplification device may be placed along the optical feeder fiber close to the remote node. Within the reach extender, the upstream signals and the downstream signals are separated from each other. The upstream signals are then amplified by a first optical amplifier within the reach extender while the downstream signals are then amplified by a different, second optical amplifier within the reach extender. Usually, the first optical amplifier causes an amplification of the upstream signal by a gain value that is similar to a gain value of an amplification of the downstream signal caused by the second amplifier. Subsequent to the amplification, the upstream and downstream signals are provided for optical transmission in their respective transmission directions.